The so-called “lost-foam” casting process is a well-known technique for producing metal castings wherein a fugitive, pyrolizable, polymeric, foam pattern, together with attached gating, runner and sprue systems (hereafter pattern assembly) is covered with a thin (i.e. 0.25-0.5 mm), gas-permeable, refractory (e.g. mica, silica, alumina, alumina-silicate, etc.) coating/skin, and embedded in a granular molding media (e.g. unbonded sand) to form a pattern-filled, mold cavity within the sand. Molten metal (hereafter “melt”) is then introduced into the pattern-filled mold cavity to pyrolyze, and displace the pattern assembly with melt. Gaseous and liquid decomposition/pyrolysis products escape through the gas-permeable, refractory skin into the interstices between the unbonded sand particles. The thickness of the refractory skin affects coating permeability, which, in turn, controls the rate at which foam decomposition/pyrolysis products are removed from the mold cavity. Typical fugitive polymeric foam patterns comprise expanded polystyrene foam (EPS) for aluminum castings, and copolymers of polymethylmethacrylate (PMMA) and EPS for iron and steel castings. A particularly effective copolymer for iron and steel comprises, by weight, 70% EPS and 30% PMMA (i.e. 70130 EPS/PMMA).
The polymeric foam pattern is made by injecting pre-expanded polymer beads into a pattern mold to impart the desired shape to the pattern. For example, raw expandable polystyrene (EPS) beads (cα. 0.2 to 0.5 mm in diameter), containing a blowing/expanding agent (e.g. n-pentane), are: (1) first, pre-expanded at a temperature above the softening temperature of polystyrene and the boiling point of the blowing agent; and (2) then, molded into the desired configuration in a steam-heated pattern mold which further expands the beads to fill the pattern mold. Complex patterns and pattern assemblies are made by molding several individual mold segments, and then gluing them together to form the finished pattern/assembly.
The melt may be either gravity-cast (i.e. poured from an overhead ladle or furnace), or countergravity-cast (i.e. forced upwardly by vacuum or low pressure into the mold cavity from an underlying vessel, e.g. a furnace). In gravity-cast lost-foam processes, the hydraulic head of the melt is the driving force for filling the mold cavity with melt. In countergravity-cast lost-foam processes, the driving force for filling the mold cavity is the intensity of the vacuum applied to the mold or the pressure applied to the melt underlying the mold.
Gravity-cast, lost-foam processes are known that: (1) top-fill the mold cavity by pouring the melt into a basin overlying the pattern so that the melt flows downwardly into the mold cavity through a gating system (i.e. one or more gates) located above the pattern; (2) bottom-fill the mold cavity by pouring the melt into a vertical sprue that lies adjacent the pattern and extends from above the mold cavity to the bottom of the mold cavity for filling the mold cavity from beneath through a gating system located beneath the pattern so that the melt flows vertically upwardly into the mold; and (3) side-fill the mold cavity by pouring the melt into a vertical sprue that lies adjacent the pattern and extends from above the mold cavity to the side of the mold cavity for horizontally filling the mold cavity through a gating system located at the side of the pattern.
The casting rate (i.e. the rate at which the metal enters the mold cavity) is limited by the rate the advancing melt front can pyrolyze the pattern and displace it from the cavity. Faster casting rates are desirable because less heat is lost from the melt during the filling process, and shorter production cycle times are possible. Shorter cycle times improve the economics of the process, while less heat loss keeps the melt hotter. Hotter melts reduce the formation of “folds” (i.e. pyrolysis products trapped at the confluence of cold metal fronts) in the casting, as well as cold-shut defects (i.e. metal that does not completely fill the pattern due to premature solidification). Casting rates have heretofore been increased by providing one or more melt flow-channels (a.k.a. “lighteners”) that extend from the gating system into the pattern, and through which the melt can rush into the pattern. Such flow-channels/lighteners typically extend into the innards of the pattern along the joints where the individual pattern segments are joined, and are molded into the pattern segments at the time the segments are formed. Such channel-forming techniques have heretofore only been effective with thicker (i.e. ≧8 mm) sections of pattern. Alternatively, the pattern segment may be molded around a narrow rod that is subsequently withdrawn from the segment to form the flow-channel. This technique is limited to forming straight flow-channels without any intervening features (e.g. turns), and hence has limited usefulness.